Substituent and environmental factors influencing photoluminescence from derivatives of bis(1,10-phenanthroline) copper(I)

Robert Michael Everly, Purdue University

Abstract

Spectroscopic analysis of the charge-transfer excited states of Bis(1,10-phenanthroline) copper(I) derivatives under a variety of experimental conditions have provided valuable new insights into the electronic nature of these systems. The absorption spectra at low temperature typically resolve into three charge-transfer bands in the visible region. With the aid of excitation and emission polarization measurements, it has been possible to make detailed group theoretical assignments in D$\sb{\rm 2d}$ symmetry. The main absorption band is z-polarized in accordance with Mulliken's theory of charge-transfer, while the higher energy transition is the first reported xy-polarized transition identified in these systems. The same polarization analysis revealed that the two states responsible for the unusual temperature dependent emission are a lower energy triplet and higher-energy singlet derived from different electronic configurations. Emission spectra and lifetime data clearly demonstrate that the steric demands of ligand substituents near the metal center have a dramatic influence upon the excited state's photochemical and photophysical behavior. For example, in a frozen matrix at 90K, complexes with larger, bulkier substituents emit at significantly higher energy and have longer excited state lifetimes and higher emission quantum yields, in some cases, two orders of magnitude greater, than complexes containing smaller substituents. This has been rationalized in terms of the difference in energy of the D$\sb{\rm 2d}$ ground state and D$\sb2$ excited state. Complexes with smaller substituents experience greater distortions which promote more facile nonradiative decay. Bulky substituents have an even greater influence in fluid media, because they additionally sterically hinder the approach of Lewis bases to the metal center preventing formation of an excited state complex (exciplex) which quenches excited state. This effect is quite evident in the concentration dependent lifetimes reported herein. In this instance, the excited state is dynamically quenched by exciplex formation with the counter anion. The lifetime reaches a limiting value indicating that molecular and solvent reorganization are the rate limiting steps in the quenching process.

Degree

Ph.D.

Advisors

McMillin, Purdue University.

Subject Area

Chemistry

COinS